In instrumentation devices such as the gauges commonly used in vehicle instrumentation panels, a pointer is rotated about an axis to indicate a measure of a parameter. In many applications of such devices, it is desirable that the pointer have a return-to-zero function. The return-to-zero function operates so that when power is not applied to the instrumentation device, the pointer returns to a zero or other nominal position. One common method for achieving the return-to-zero pointer function is to use a coil spring that provides a minimum torque, which returns the pointer to the zero position in the absence of a supervening signal. In another common method, one or more biasing magnets are placed proximate to a rotor of the apparatus. The biasing magnets provide a biasing magnetic field that, in the absence of a supervening signal, acts on the magnetic rotor to cause the rotor to turn until the pointer reaches the zero or nominal position. Both of these methods require additional parts in the apparatus in the form of either magnets or springs causing an increased price to obtain a gauge with a return-to-zero function.
An example prior art apparatus requiring biasing magnets is shown in FIG. 1. The apparatus shown includes a bobbin assembly 16 comprising upper bobbin 18 and lower bobbin 20. The bobbin assembly 16 defines a cavity in which rotor 32 is mounted on spindle 30 by bushing 31. The spindle 30 is rotatably mounted in bearings 34 and 36 in the bobbin assembly so that, as magnetic forces operate on rotor 32, it freely rotates, forcing rotation of spindle 30 to which pointer 38 is attached at the spindle end 40. Two coils, 26 and 28, are wound around the bobbin assembly 16 substantially perpendicular to each other and those coils are energized through leads 22, of which there are typically four (only two shown). The leads 22 are electrically connected to the coils 26 and 28 at connectors 24.
Within the cavity defined by bobbin assembly 16 where the rotor 32 is placed, a dampening fluid such as a silicone gel may be placed to dampen rotational movement of the rotor 32 and spindle 30. The instrumentation apparatus is typically placed in a low carbon steel can 46 such that the end 40 of spindle 30 is at the open end 48 of the can 46. Can 46 operates to block stray magnetic fields from operating on the rotor 32.
To provide a return-to-zero function for the instrumentation apparatus shown, two pre-formed magnets 14 are placed in retainers 12 and attached to the inside of can 46 proximate to coil 28 in the regions 42 and 44 indicated. The magnets 14 provide the magnetic field across the rotor 32 that tends to return the rotor 32, spindle 30 and pointer 38 to a nominal position, for example the zero position, when no magnetic fields caused by coils 26 and 28 are operating on the rotor 32.
In other implementations, the apparatus shown in FIG. 1 may be constructed with higher intensity magnets as magnets 14 so that the apparatus may be used as a volt gauge. When the apparatus is used as a volt gauge, the magnetic field created by the higher intensity magnets in combination with the field created by the coils 26 and 28 orients the rotor 32 to a position dependent upon the voltage across the coils 26 and 28.